.\" -*- nroff -*-
.\" Copyright (c) 2010-2014 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines
.\" Copyright (c) 2020      Google, LLC. All rights reserved.
.\" $COPYRIGHT$
.TH MPI_Buffer_attach 3 "#OMPI_DATE#" "#PACKAGE_VERSION#" "#PACKAGE_NAME#"
.SH NAME
\fBMPI_Buffer_attach\fP \- Attaches a user-defined buffer for sending.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Buffer_attach(void \fI*buf\fP, int\fI size\fP)

.fi
.SH Fortran Syntax
.nf
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_BUFFER_ATTACH(\fIBUF\fP,\fI SIZE\fP, \fIIERROR\fP)
	<type>	\fIBUF\fP(\fI*\fP)
	INTEGER	\fISIZE\fP,\fI IERROR \fP

.fi
.SH Fortran 2008 Syntax
.nf
USE mpi_f08
MPI_Buffer_attach(\fIbuffer\fP, \fIsize\fP, \fIierror\fP)
	TYPE(*), DIMENSION(..), ASYNCHRONOUS :: \fIbuffer\fP
	INTEGER, INTENT(IN) :: \fIsize\fP
	INTEGER, OPTIONAL, INTENT(OUT) :: \fIierror\fP

.fi
.SH INPUT PARAMETERS
.ft R
.TP 1i
buf
Initial buffer address (choice).
.TP 1i
size
Buffer size, in bytes (integer).

.SH OUTPUT PARAMETER
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
Provides to MPI a buffer in the user's memory to be used for buffering outgoing messages. The buffer is used only by messages sent in buffered mode. Only one buffer can be attached to a process at a time.

.SH NOTES
.ft R
The size given should be the sum of the sizes of all outstanding Bsends that you intend to have, plus MPI_BSEND_OVERHEAD bytes for each Bsend that you do. For the purposes of calculating size, you should use MPI_Pack_size. In other words, in the code
.sp
.nf
    MPI_Buffer_attach( buf, size );
    MPI_Bsend( \&..., count=20, datatype=type1, \&... );
    \&...
    MPI_Bsend( \&..., count=40, datatype=type2, \&... );
.fi
.sp
the value of size in the MPI_Buffer_attach call should be greater than the value computed by
.sp
.nf
    MPI_Pack_size( 20, type1, comm, &s1 );
    MPI_Pack_size( 40, type2, comm, &s2 );
    size = s1 + s2 + 2 * MPI_BSEND_OVERHEAD;
.fi
.sp
MPI_BSEND_OVERHEAD gives the maximum amount of buffer space that may be used by the Bsend routines. This value is in mpi.h for C and mpif.h for Fortran.

.SH ERRORS
Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

.SH SEE ALSO
.ft R
.sp
MPI_Buffer_detach

